> You could write XHTML 1.0 documents that were backwards compatible to browsers that only understood HTML 4.01. You could and a lot of people _tried_, or at l

> You could write XHTML 1.0 documents that were backwards compatible to browsers that only understood HTML 4.01. You could and a lot of people _tried_, or at least pretended to. But the vast majority of documents that tried to do this failed to actually be well-formed XML, for various reasons... In practice, even restricting parsing as XML to cases when the page was explicitly sent with the application/xhtml+xml MIME type would leave a browser with problems when sites sent non-well-formed XML with that MIME type. This was a pretty serious problem for Gecko back in the day when we attempted to push XHTML usage (e.g. by putting "application/xhtml+xml" ahead of "text/html" in the Accept header). So we stopped pushing that, since it was actively harming our users...